Retroreflective traffic stripe

ABSTRACT

A retroreflective traffic stripe comprising a widely spaced repeating pattern of linear light-turning prisms over cube-corner retroreflective prisms. The light-turning prisms comprise at least two exposed surfaces: one approximately vertical facing the headlights of an oncoming vehicle, and another opposing the first and sloped by approximately 45 degrees. The approximately vertical surface accepts light from the headlights and transmits such light to the sloped surface which totally internally reflects such light downward onto an array of cube corner retroreflective prisms, which totally internally reflect such light in approximately the reverse direction. Such reflected light once more encounters the sloped face of the light turning prisms which totally internally reflects the light toward the approximately vertical surface, where such light exits and returns toward the headlights and toward the eyes of the driver of the vehicle.

CROSS REFERENCE TO RELATED APPLICATIONS

Two provisional patent applications were previously filed with the U.S.Patent and Trademark Office by the inventor each disclosing key elementsof the present invention. Application No. 62859271, entitled“Retroreflective Material for Horizontal Road Markings Comprising LinearLight Turning Prisms over Cube Corner Retroreflective Prisms,” was filedJun. 10, 2019. Application No. 62/930,821, entitled “OptimalConfigurations of Retroreflective Material for Horizontal Road MarkingsComprising Linear Light Turning Prisms over Cube Corner RetroreflectivePrisms,” was filed Nov. 5, 2019. The inventor claims the respectivefiling dates of these provisional applications for the key elementsdisclosed in these respective provisional applications.

BACKGROUND OF THE INVENTION

This invention relates to retroreflective traffic stripes which areilluminated at night by the headlights of vehicles (cars, SUVs, trucks,and motorcycles) and which return a portion of the incident illuminationby reflection to the drivers of these vehicles. Such traffic stripes areapplied to or attached to the substantially horizontal surfaces ofhighways and widely used as (1) longitudinal lane delineation markersparallel to the lanes of traffic between lanes, (2) longitudinal roadedge markers also parallel to the lanes of traffic, and (3) lateralmarkers perpendicular to the lanes of traffic at crosswalks andintersections. This invention further relates to retroreflective trafficstripes applied to the substantially vertical surfaces of guardrails andconcrete barriers.

For present retroreflective traffic stripes used on road surfaces, thebest approach to date is to embed glass or ceramic beads in the topsurface of the white or yellow paint to achieve a small amount ofretroreflection of the illumination of headlights back toward the driverof the vehicle. These beads are not very effective, especially in wetweather, and are easily broken or dislodged from the top surface of thepaint by traffic running over them or by snowplows in colder climates.For present retroreflective traffic stripes used on guardrails andconcrete barriers, the best approach to date is to deform the stripes toprovide regions which stick outward from the guardrails or concretebarriers to provide a better incidence angle for the headlight beams.These deformed stripes are expensive, and installation islabor-intensive.

What is needed to provide exceptionally bright traffic stripes is a newoptical material which can efficiently accept high incidence angle lightfrom approaching headlights, turn this light by about 90 degrees, andsend this light on to an array of cube corner retroreflective prisms,which then efficiently reverses the light path and sends the light backto the driver of the vehicle. The present invention elegantly fulfillsthis need, as described in the following paragraphs. By innovativelycombining a top layer of light turning prisms with a bottom layer ofprismatic cube corner retroreflective prisms in a critical optimalconfiguration, an extremely efficient, moderate cost new material fortraffic stripes has been invented, which could save many lives on thehighways of the world. The new traffic stripe offers 1,000 times theretroreflective brightness of conventional traffic stripes.

BRIEF SUMMARY OF THE INVENTION

This invention is a novel retroreflective traffic stripe comprising awidely spaced repeating pattern of linear light turning prisms over cubecorner retroreflective prisms in a critical optimal configuration. Thelight turning prisms comprise at least two exposed surfaces, oneapproximately vertical facing the headlights of oncoming traffic, andanother opposing the first and sloped by approximately 45 degrees. Theapproximately vertical surface efficiently accepts light from theheadlights and transmits such light to the sloped surface which reflectssuch light downward whereafter such light intercepts an array of cubecorner retroreflective prisms, which reflects such light upward inapproximately the reverse direction. Such reflected light once moreencounters the sloped face of the light turning prisms and is reflectedtoward the approximately vertical surface, whereafter such light exitsand returns toward the headlights and, more importantly, toward the eyesof the driver of the vehicle.

The invention employs an array of cube corner retroreflective prismsbelow a widely spaced repeating pattern of light turning prisms in aunique optimized configuration to provide a new type of retroreflectivetraffic stripe, with 1,000X greater brightness than the current state ofthe art. The invention uses polymer micro-prismatic sheet made bywell-established, high-speed, cost-effective, roll-to-roll embossingprocesses. The leading candidate materials for the micro-prismatic sheetare transparent robust polymers such as acrylic, polycarbonate,polyurethane, silicone, fluoropolymer, and combinations thereof.

The invention requires no metallization of the surfaces of the lightturning prisms or the cube corner prisms, which perform theirreflections using the well-known phenomenon of total internal reflection(TIR). TIR is ensured by using polymer materials with refractive indicesabove 1.4 and by surrounding the outside surfaces of the prisms withair. The candidate materials inherently meet the refractive indexrequirements (1.49 for acrylic, 1.58 for polycarbonate, 1.52 forpolyurethane, 1.41 for silicone, 1.40 fluoropolymers such as ETFE).

The invention may be produced in various ways. The light turning prismsand the cube corner retroreflective prisms may be embossed ontoindividual microstructured polymer sheets and laminated together with atransparent adhesive, or they may be embossed on opposite sides of thesame sheet of polymer.

The invention may use transparent films between the linear light turningprisms and the cube corner retroreflective prisms to impede moisturepenetration into the retroreflective prisms.

The invention may also use films below the cube corner retroreflectiveprisms to impede moisture penetration and thereby create a dry aircavity in contact with the cube corner retroreflective prisms, therebyenabling such prisms to perform their reflective function using totalinternal reflection rather than metallic reflection. Alternately, thesecube corner retroreflective prisms may be metallized with aluminum orsilver or other metal to perform their reflective function usingmetallic reflection.

The invention may use either total internal reflection or metallicreflection on the sloped surfaces of the linear light turning prisms.

The invention may use an adhesive layer beneath the other layers tofacilitate bonding of the material to the roadway surface.

The invention may be made in roll form to enable machine-aidedcontinuous application of road stripes to long sections of highways androads.

The invention may include a light turning prism configuration with athird surface connecting the vertical surface and the 45-degree slopedsurface of the prism to provide traffic damage mitigation.

The invention may include features that protrude above the tops of thelight turning linear prisms to resist the pressure of traffic tires andsnowplows and thereby protect and prevent damage to the light turningprisms.

The invention may be used in a vertical orientation for placement onguardrails or walls next to the highway.

The invention may be used with light turning prismatic features facingopposite directions to be visible from vehicles traveling in bothdirections.

The invention may include a white back film to provide daytimebrightness for the road stripe.

The invention may include colored pigment in the prismatic polymermaterial, such as yellow, red, or other colors to impart color to theretroreflected rays from headlights at night, or to the reflected raysfrom sunlight during the day.

The invention may include raised shoulders on the two long edges of thetraffic stripe to further mitigate traffic damage.

The invention may include compliant layers beneath the road stripe tofurther mitigate traffic damage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the present invention in multiple views. FIG. 1-A shows theinvention from a driver's perspective for a longitudinal road stripeapplication such as a lane delineation marker or road edge marker, bothof which extend lengthwise parallel to the direction of traffic. FIG.1-B shows a cross-sectional view of a small portion of the invention,demonstrating the optical operation of the invention. FIG. 1-C shows aview of the invention for a longitudinal road stripe application. FIG.1-C shows a view of the invention for a lateral road stripe application.For FIG. 1 and the remaining figures, the numerals shown in the drawingsrepresent these key elements of the invention:

-   -   1 Light Turning Prism(s)    -   2 Cube Corner Prism(s)    -   3 Structural Bar(s)    -   4 Incident Rays    -   5 Retroreflected Rays    -   6 Raised Shoulders    -   7 Gaps for Rain Runoff    -   8 Compliant Base    -   9 Missing Corners    -   10 Dirt in Valleys

FIG. 1-A shows the widely spaced repeating light turning prisms 1extending across the traffic stripe.

FIG. 1-A also shows the raised shoulders 6 which mitigate traffic damageto the traffic stripe by contacting the tires of any vehicle which runsover the road stripe before such tires can contact the other componentsof the road stripe.

FIG. 1-A also shows gaps 7 in the shoulders 6 to allow rainwater run-offto the edge of the lane or road thereby minimizing accumulation of wateron top of the traffic stripe.

FIG. 1-A also shows a compliant base 8 under the traffic stripe tofurther mitigate traffic damage.

FIG. 1-B shows the key elements of the invention and explains how itoperates optically. Rays 4 from distant headlights enter the verticalface of the light turning prism 1 and proceed to intercept the slopedsurface of the light turning prism 1, which reflects these raysdownward. These rays then proceed to intercept an array of cube cornerretroreflective prisms 2, which redirect the rays back upward in theopposite direction from which the rays arrived. The rays then againintercept the sloped surface of the light turning prism 1, whichreflects the rays 5 back in the opposite direction from which the rays 4initially arrived.

FIG. 1-B also shows some critical relative dimensions of the newinvention. The cube corner prisms 2 must be much smaller in extent thanthe light turning prism 1 in the lengthwise direction of the trafficstripe shown as left to right in FIG. 1-B. For the preferred embodimentof FIG. 1-B, there are eight cube corner prisms 2 under one lightturning prism 1, and six of these cube corner prisms 2 are illuminatedby rays 4 which are retroreflected as rays 5 in the opposite direction.As will be fully explained later in this application, the new inventionwill not provide acceptable optical performance unless the cube cornerprisms 2 are much smaller than the light turning prisms 1. As will alsobe fully explained later in this application, the new invention will notprovide acceptable optical performance unless a substantial fraction ofthe cube corner prisms 2 are illuminated by the rays 4. To enable suchillumination of a substantial fraction of the smaller cube corner prisms2 by each of the larger light turning prisms 1, the lengthwise spacingbetween light turning prisms 1 must be large enough to prevent blockageof light from the distant headlights by neighboring light turning prisms1, such that unblocked light can be delivered to a substantial fractionof the cube corner prisms 2.

FIG. 1-B also shows a structural element 3 which is taller than thelight turning prism 2 to provide traffic damage mitigation, sincevehicle tires will intercept the structural element 3 beforeintercepting light turning prism 2.

FIG. 1-B also shows a second light turning prism 1 on the left facing inthe opposite direction from the light turning prism 1 on the right toprovide bi-directional functionality for the traffic stripe. Thisconfiguration allows the traffic stripe to be equally visible fortraffic moving in both directions along the highway, shown as left andright in FIG. 1-B.

FIG. 1-C shows a top isometric view of the new traffic stripe configuredfor longitudinal applications parallel to the road, such as lanedelineation stripes and road edge stripes. The repeating pattern oflight turning prisms 1 are spaced apart along the length of the trafficstripe, which is typically 10 cm (4 inches) wide.

FIG. 1-D shows a top isometric view of the new traffic stripe configuredfor lateral applications perpendicular to the road, such as crosswalkstripes or intersection stripes. The repeating pattern of light turningprisms 1 are spaced apart along the short 10 cm (4 inch) length of thetraffic stripe, which typically has a width as wide as the road.

FIG. 2 shows a set of three-dimensional views of the opticallyfunctional prisms previously shown in cross section in FIG. 1-B. FIG. 2clarifies the operation of the invention in three dimensions.

FIG. 2-A shows a transparent top view of the light turning prism 1, thearray of cube corner prisms 2, the structural bar 3, and the incidentrays 4 and reflected rays 5 which overlap one another while proceedingin their opposite directions. The ray paths have been calculatedrigorously by the inventor.

FIG. 2-B shows an isometric top view of the light turning prism 1, thearray of cube corner prisms 2, and the structural bar 3, with no raysshown.

FIG. 2-C shows a transparent bottom view of the light turning prism 1,the array of cube corner prisms 2, the structural bar 3, and theincident rays 4 and reflected rays 5 which overlap one another whileproceeding in their opposite directions. The ray paths have beencalculated rigorously by the inventor.

FIG. 2-D shows an isometric bottom view of the light turning prism 1,the array of cube corner prisms 2, and the structural bar 3, with norays shown.

FIG. 3 shows the present invention in additional cross-sectional viewsfor incident rays 4 coming from distant headlights located to the rightof the views.

FIG. 3-A shows two adjacent sets of light turning prisms 1 andstructural bars 3 spaced widely apart along the length of the trafficstripe. The spacing has been selected to prevent incident ray blockageby the set of prisms on the right for light heading toward the set ofprisms on the left to fully illuminate six cube corner prisms 2 out ofeight total cube corner prisms 2 under one light turning prism 1.Incident rays 4 and reflected rays 5 follow the same paths in oppositedirections.

FIG. 3-B shows the paths of the rays 4 and 5 for the set of prisms onthe left, including a blow-up view of the cube corner prisms 2.

FIG. 3-C shows the paths of the rays 4 and 5 above the set of prisms onthe right. No ray blockage occurs by the structural bar 3 for thedesired rays 4 and 5 because the spacing between adjacent sets of prismson the left and right has been carefully chosen.

FIG. 4 shows the same cross-sectional views of the invention as FIG. 3,but for incident rays 4 coming from distant headlights located to theleft of the views. The optical performance is the same for lightarriving from either direction. FIGS. 4-A, 4-B, and 4-C present similarviews as FIGS. 3-A, 3-B, and 3-C.

FIGS. 5-8 present the results of a parametric ray trace analysis by theinventor to fully understand and to optimize the configuration of theinvention. The incident rays 4 arrive at a grazing angle of 1.24 degreesoff horizontal, corresponding to illumination by headlights 0.65 metersabove the road and 30 meters away. Each figure has four views, labeled-A, -B, -C, and -D. The A and B views correspond to full illumination ofthe light turning prism 1 from top to bottom of the vertical face. Suchfull illumination would require a spacing between sets of light turningprisms such that no blockage occurred by adjacent prisms of rays fromdistant headlights proceeding to the vertical face of the light turningprism 1. The C and D views correspond to half illumination of the lightturning prism 1, namely, the top half of the vertical face. Such halfillumination would require a spacing between sets of light turningprisms such that no blockage occurred by adjacent prisms of rays fromdistant headlights proceeding to the top half of the vertical face ofthe light turning prism 1.

The A and C views of FIGS. 5-8 correspond to perfectly aligned cubecorner prisms 2 under light turning prisms 1. The B and D views of FIGS.5-8 correspond to perfectly misaligned cube corner prisms 2 under lightturning prisms 1.

For the perfectly aligned cases shown in the A and C views of FIGS. 6-8,all of the incident rays 4, shown with a diagonal hatch pattern fromupper left to lower right, are successfully retroreflected in thereverse direction as reflected rays 5, shown with a diagonal hatchpattern from lower left to upper right, and no rays are lost by blockageor misdirection. The overlapping incident rays 4 and reflected rays 5are shown by the overlapping hatch patterns. One exception to this ruleis shown in FIG. 5-C. When only half of the vertical face of the lightturning prism 2 is illuminated and only one aligned cube corner cubecorner prism is used below the tilted surface of the light turning prism1, all of the retroreflected light is lost by blockage by the nextadjacent set of light turning prisms 1 and structural bar 3. Thiscomplete loss of performance for a half-illuminated light turning prism1 over a single cube corner prism 2 under the tilted surface of lightturning prism 1 is due to a unique optical feature of cube cornerprisms. Light that enters the right half of the cube corner prism 2departs from the left half of cube corner prism 2, and vice versa. Thisleads to an offset of the reflected ray bundle compared to the incidentray bundle equal to one half the length of the cube corner prism 2. FIG.5-C shows that the incident rays 4 enter the cube corner prism 2 on theright side and leave on the left side, causing an offset in thereflected rays 5. This offset further leads to the bundle of reflectedrays departing the light turning prism 1 through the bottom half of thevertical surface of the light turning prism 1. This bundle of reflectedrays 5 will never make it back to the vehicle due to complete blockageby the next adjacent set of light turning prisms 1 and structural bars3.

For the perfectly misaligned cases shown in the B and D views of FIGS.5-8, a significant fraction of the incident rays 4, shown with adiagonal hatch pattern from upper left to lower right, areunsuccessfully retroreflected in the reverse direction as reflected rays5, shown with a diagonal hatch pattern from lower left to upper right,and many rays are lost by blockage or misdirection. The successfullyretroreflected overlapping incident rays 4 and reflected rays 5 areshown by the overlapping hatch patterns. The fraction of incident rays 4which are lost is greatest for fewer cube corner prisms 2 below thesloped surface of the light turning prism 1. The fraction of incidentrays 4 which are lost is greater for half illumination of the lightturning prisms than for full illumination of the light turning prisms 2.Thus, the lost ray fraction is greater for the D views than for the Bviews and the lost ray fraction is greater for FIG. 5 than for FIG. 6,and greater for FIG. 6 than for FIG. 7, and greater for FIG. 7 than forFIG. 8.

Inspection of the results in FIGS. 5-8 for the misaligned cube cornerprisms 2 shows two types of lost reflected rays. The first type of lostrays corresponds to reflected rays which escape the prismatic structuresubstantially vertically. The second type of lost rays corresponds toreflected rays which leave the light turning prism in the correctdirection but too low, such that they will be blocked by the adjacentlight turning prism 1 and taller structural bar 3, never reaching thevehicle. For example, FIG. 6-B shows 50% lost ray fraction due tovertical escape of reflected rays, while FIG. 6-D shows 100% lost rayfraction due to 50% vertical ray escape and 50% too low reflected rays5.

FIG. 9 summarizes the results of the parametric ray trace analysiscorresponding to FIGS. 5-8 in graphical form. The abscissa of each graphin FIG. 9 is the total number of cube corner prisms 2 below each lightturning prism 1. The ordinate of each graph in FIG. 9 is the fractionalloss of reflected rays 5 corresponding to incident rays 4. Theworst-case fractional loss of reflected rays 5 compared to incident rays4 is shown in FIG. 9-A, and corresponds to perfectly misaligned cubecorner prisms 2 under light turning prism 1. The top curve of FIG. 9-Acorresponds to half illumination of light turning prism 1, and thebottom curve of FIG. 9-A corresponds to full illumination of lightturning prism 1.

Since the loss is zero for perfectly aligned cube corner prisms 2 underlight turning prisms 1, for a wide variation in cube corner prism 2alignment with light turning prism 1, from perfectly aligned toperfectly misaligned, the average loss would be expected to be aboutone-half of the worst case loss, as plotted in FIG. 9-B. Such a widevariation in alignment of prisms 1 and 2 is fully anticipated formass-produced prismatic sheet material due to a variety of phenomena,including master tool diamond turning tolerances, production toolreplication and assembly tolerances, differential thermalexpansion/contraction of the tooling and polymer sheet during thethermal embossing process, shrinkage of the polymer from hot moltenstate to cool solid state, and cumulative tolerances along the length ofa continuous roll of prismatic sheet material. The inventor has workedfor several decades with world-class providers of diamond-turned mastertools, electroform replicated production tooling drums, and roll-to-rollthermal embossing processing of polymer film into microstructuredprismatic sheet. Precise alignment of all the cube corner prisms 2 withall the light turning prisms 1 for prismatic sheet material produced inrolls hundreds of meters long is not practical. Therefore, theconfiguration of the present invention needs to accommodate widevariation in alignment and misalignment.

The average loss shown in FIG. 9-B is about half as much for fullillumination of the light turning prisms 1 compared to half illuminationof the light turning prisms 1. However, full illumination is notdesirable from practical considerations for the traffic stripeapplication. As shown in FIG. 10, dirt accumulation in the valleys nextto the light turning prisms 1 will cause optical losses over the lowerportion of the light turning prisms 1. Partial illumination of the lightturning prism 1 can avoid the use of this lower portion of the lightturning prism 1, mitigating dirt losses for the new traffic stripe.

To maximize the optical performance of the invention, the inventor seeksto keep the lost ray fraction below 10% by selecting a preferredembodiment shown by the X data point in FIG. 9-B. This preferredembodiment corresponds to eight cube corner prisms 2 under each lightturning prism 1. This preferred embodiment further corresponds to 75%illumination of the light turning prism 1, such that six cube cornerprisms 2 will be illuminated. The corresponding average fractional rayloss is seen to be about 8% in FIG. 9-B.

FIG. 10 shows more rationale for the preferred embodiment of the presentinvention.

FIG. 10-A shows a pristine set of light turning prisms 1 correspondingto a newly installed traffic stripe. FIG. 10-B shows a damaged and dirtyset of light turning prisms corresponding to a traffic stripe after muchtime in the field. The sharp corners on the light turning prisms 1 andthe structural bar 3 have been worn down and are shown with missing androunded corners 9. Dirt 10 has accumulated in the valleys on either sideof the light turning prisms 1. Despite these changes, the reflected rays5 still correspond to about two-thirds of the reflected rays 5 for thepristine sample. Therefore, if the pristine sample in FIG. 10-A provides1,000X brighter retroreflectivity than current state of the art trafficstripes, as will be shown later, the damaged and dirty sample in FIG.10-B will provide about 667X brighter retroreflectivity than currentstate of the art traffic stripes.

FIG. 11 shows the basic geometrical parameters which together describethe invention. The repeating pattern of light turning prisms 1 andstructural bars 3 are spaced apart by spacing dimension S as shown inFIG. 11-A. The geometry involved in preventing blockage of incident rays4 and reflected rays 5 is shown in FIG. 11-B. The number of illuminatedcube corner prisms 2 is defined in FIG. 11-C. The general dimensions forthe prismatic and structural elements of the invention are defined inFIG. 11-D. While the preferred embodiment has eight cube corner prisms 2under the sloped surface of light turning prism 1 with six of these cubecorner prisms 2 are illuminated, the inventor has established slightlyless demanding design rules shown in FIG. 11 to bracket the generaldimensions of the invention for good optical performance. The reason forusing the angle 1.24° in the design rule for spacing S is furtherexplained below.

FIG. 12 shows the actual dimensions of the preferred embodiment of theinvention. The inventor selected these dimensions to enable the massproduction of the embossed polymer sheet with light turning prisms 1 onone side and cube corner prisms 2 on the other side while starting witha polymer sheet about 0.075 mm thick before embossing. The inventor hasconfirmed with leading providers of tooling and roll-to-roll prismaticsheet embossing that these dimensions are practical for low-cost massproduction of the invention.

FIG. 13 describes the present state of the art in traffic stripes andshows examples of the retroreflective brightness. The Federal HighwayAdministration (FHWA) has proposed a rule that the minimum brightness ofsuch traffic stripes should be 100 mcd/m²-lux for all roadways in theUnited States with speed limits of 70 miles per hour and above. Manystates have adopted this same standard. As shown by the photos in FIG.13, this minimum brightness is not very bright. For comparison, thetable in FIG. 13 shows the required retroreflective brightness of roadsigns to meet the FHWA standard for Type XI road sign sheeting. Thevalues in the table are for different observation angles relative to thesign, and all of these values are more than 1,200X greater than theproposed value for traffic stripes discussed above. The new inventionmerely changes the direction of the incident rays 4 to direct these raysonto cube corner retroreflective prisms exactly like those used in TypeXI road sign sheeting. Therefore, the reflected rays 5 will have thesame retroreflective brightness for the new traffic stripe as for TypeXI road sign sheeting except for fractional losses in reflected rayscompared to incident rays. As discussed above, the preferred embodimentof the invention keeps such losses below 10%, enabling theretroreflective brightness to remain more than 1,000X greater than theminimum target for traffic stripes set in the proposed FHWA rule. Thisexceptional brightness will minimize lane departure accidents and savelives, especially for older drivers with reduced visual acuity.

The new traffic stripe can not only be used as road stripes, includinglane delineation marking stripes and road edge stripes and crosswalkstripes, but also as retroreflective traffic stripes on verticalconcrete barriers and guard rails, as shown in FIG. 14. For suchapplications, the invention can be made in tape form and quickly andinexpensively applied to the concrete barriers and guardrails using apressure sensitive adhesive to bond the traffic stripe to the concreteor metal structure, much like adhesive tape.

FIG. 15 shows the standard method of measuring traffic striperetroreflectivity, documented in ASTM 1710 and equivalent internationalstandards. The light is incident from headlights which are 0.65 meterabove the road and 30 meters from the test location of the road stripe.The arctangent of 0.65 meter/30 meters=1.24°, establishing the designvalue previously shown in FIG. 11. This grazing angle of incidence isthe reason that high retroreflectivity is so difficult to achieve fortraffic stripes. One can easily compare the brightness of road signs toroad stripes in a nighttime drive to experience the 1,000X higherbrightness of the signs compared to the stripes. The test standard inFIG. 15 assumes the driver of the vehicle is 1.2 meters above the road.Standard test equipment is available from many vendors to simulate thegeometry of FIG. 15 and make such retroreflectivity measurements. FIG.15 also references the FHWA proposed design rule.

The mass production and application of cube corner retroreflectivesheeting in road signs are well established technologies with manydecades of history. Leading firms in this market area include 3M andAvery Dennison. Both firms have refined such cube corner retroreflectivesheeting technology through multiple generation over multiple decades.The latest and brightness retroreflective sheeting product families areknown as Diamond Grade 3 by 3M and Omnicube by Avery Dennison. Bothfamilies of products meet or exceed the retroreflective brightnessstandards for FHWA Grade XI sheeting previously shown in FIG. 13. Bothfamilies of products use an array of cube corners of a sophisticatedconfiguration known in the industry as “full cube” retroreflectiveprisms. FIG. 16 explains this “full cube” technology in more detail.

A complete cube corner has three orthogonal faces which form atriangular aperture as shown in FIG. 16-A. When this aperture isilluminated, only two-thirds of it accepts and reflects incident raysfrom all three faces, sending the rays back toward the source ofillumination. The other one-third of the rays only reflects the incidentrays from two faces, not sending the rays back toward the source of theillumination. FIG. 16-B shows the dead areas in black corresponding tothe non-retroreflecting portions of the complete cube corner. The liveareas which retroreflect are shown in white.

FIG. 16-C shows that a rectangular portion of the live area can betrimmed from the complete cube corner and used as a 100% retroreflective“full cube” prismatic element. To enable tooling to be made to producesuch full cube elements, two of these elements are often butted togetherto form a pair of full cube prisms. Such pairs of full cube prisms arethen arrayed together to form a retroreflective prismatic sheet with100% active area. In the preferred embodiment of the invention, suchfull cube prisms are preferred as the cube corner prisms 2 beneath thelight turning prisms 1.

FIG. 17 shows the full cube prisms, pair of such prisms, and array ofpairs of such prisms in three dimensional drawings. While the presentinvention will work with other types of cube corner prisms, the bestperformance will be achieved with full cube technology in the cubecorner prisms 2 beneath the light turning prisms 1. The previous figureshave included full cube prisms as the lower set of prisms in the variousembodiments of the invention.

FIG. 18 shows currently available full cube retroreflective sheeting forroad signs identified as Type XI. The two leading manufacturers of thisType XI sheeting are 3M and Avery Dennison. FIG. 17-A is excerpted fromthe FHWA sheet identification guide of 2014 and includes 3M's DiamondGrade 3 ® products and Avery Dennison's Omnicube® products. The whitehoneycomb pattern corresponds to the sealing bonds around individual airpockets which are required to enable total internal reflection (TIR) bythe cube corner prisms. The honeycomb patterns are slightly differentfor the 3M and Avery Dennison products, but, in both cases, they block asignificant fraction of the total area of cube corner retroreflectiveprisms. This blockage causes a significant loss in retroreflectivity onthe order of 20-30%. The air pockets must be small, on the order of afew millimeters, to allow the sheeting to be trimmed to fit a variety ofsign sizes and shapes. The trimming destroys the air pockets along thetrim lines, and therefore large air pockets would result in too muchdead area around the edges of the sign.

FIG. 18-B shows an enlarged photo of the 3M DG3 product, highlightingthe cube corner prisms which appear dark in the photo and the air pocketseals which appear light in the photo. It is apparent that the sealsblock a substantial portion of the total area of the cube corner prisms,reducing performance proportionally. For the present invention, largerair pockets with much lower optical losses could be used since trimmingis not necessary for traffic stripes.

DETAILED DESCRIPTION AND BEST MODE OF IMPLEMENTATION

The present invention is a novel retroreflective traffic stripe offeringunprecedented brightness when illuminated by headlights from vehicles onhighways. The invention comprises a thin transparent polymer sheet withprisms of two distinct types embossed onto the top and bottom surfacesof the sheet. The invention is best understood by referring to theattached drawings, which were briefly discussed in the previousparagraphs. FIG. 1 shows the basic configuration of the invention invarious views.

FIG. 1-A shows a view as would be seen by the driver of a vehicle on ahighway with the new traffic stripe mounted to the edge of the laneparallel to the direction of travel. The top surface of the trafficstripe includes light turning prisms 1 which are widely spaced along thelength of the traffic stripe and are oriented perpendicular to thetraffic stripe. These light turning prisms 1 have substantially verticalfaces which are nearly perpendicular to the incident rays 4 from theheadlights, allowing the incident rays 4 to be efficiently transmittedinto the light turning prisms 1.

FIG. 1-B shows the cross-sectional configuration of the light turningprisms 1, including the vertical faces mentioned above. The incidentrays 4 which enter the light turning prisms 1 proceed onward to a tiltedface of the light turning prism which reflects the incident raysdownward by the well-known process of total internal reflection (TIR).The tilted face is tilted at approximately 45 degrees relative to bothhorizontal and vertical directions as shown in FIG. 1-B.

The rays which are reflected downward by the light turning prisms 1proceed until these rays next intersect cube corner retroreflectiveprisms 2 on the bottom surface of the transparent polymer sheet. Theseretroreflective cube corner prisms 2 reflect the rays three successivetimes from their three orthogonal faces thereby returning the rays inthe opposite upward direction. These three reflections by the cubecorner prisms 2 also employ TIR. The reflected rays 5 next encounter thetilted surface of the light turning prism 1 which once again reflectsthe rays 5 to now direct them toward the right. These reflected raysthen exit the vertical face of the light turning prism 1 inapproximately the opposite direction of the incident rays 4.

Several critical features of the invention are also shown in FIG. 1-B.The reflective cube corner prisms 2 must be substantially smaller inlength, shown as the left to right direction, than the light turningprisms 1. For the preferred embodiment shown in FIG. 1-B, eight of thesmaller cube corner prisms 2 fit below the tilted face of light turningprism 1. In other words, the length of each cube corner prism is 87.5%smaller than the length of the tilted face in the left to rightdirection of FIG. 1-B.

To mitigate traffic damage of the light turning prisms 1, tallerstructural bars 3 are employed to encounter the surfaces of tires onvehicles traveling the roadway thereby minimizing tire contact on theshorter light turning prisms 1. In addition, the light turning prisms 1are configured with flat tops to reduce internal stresses from tirecontact within the light turning prisms 1. While pointed tops would workoptically for light turning prisms 1, such pointed tops would representhigh internal stress points under tire contact, and are not preferredfor applications with potential traffic damage.

FIG. 1-A also shows additional critical features of the new inventionwhen it is installed on a road surface. To further minimize trafficdamage, taller shoulders 6 are employed to protect the shorter prismaticsheet material comprising the new traffic stripe. Such shoulders 6 canbe made of many durable materials, such as the American Road Patch®material used to repair roads as described in U.S. Pat. Nos. 8,534,954and 8,858,115. To facilitate rainwater runoff, intermittent gaps 7 areemployed in the shoulders 6. To further mitigate traffic damage,compliant material 8 is employed between the prismatic polymer sheet andthe roadway below. The entire road stripe assembly is attached to theroad surface using one of several well-known type of adhesives,including pressure sensitive adhesive (PSA), liquid adhesives, hotadhesives, etc.

FIG. 1-C and FIG. 1-D show two different patterns of the light turningprisms 1 which are spaced apart on the top surface of the new trafficstripe. For longitudinal applications wherein the road stripe isparallel to the direction of the highway, such as lane delineationstripes and road edge stripes, the light turning prisms 1 areperpendicular to the road stripe, as shown in FIG. 1-C. For lateralapplications wherein the road stripe is perpendicular to the highway,such as crosswalk stripes and intersection stripes, the light turningprisms 1 are parallel to the road stripe, as shown in FIG. 1-C. Roadstripes are typically about 10 cm (4 inches) in the smaller dimension bymuch greater dimensions in the longer dimension, as shown in FIGS. 1-Cand 1-D.

FIG. 2 shows the optical functionality of the invention in threedimensional views. These views are for a small portion of the roadstripe comprising the light turning prisms 1, the structural bar 3, andthe cube corner prisms 2. Incident rays 4 and retroreflected rays 5 areshown in two of the views and not shown in the other two of the views.FIG. 2-A is a transparent top isometric view with rays. FIG. 2-B is atop isometric view without rays. FIG. 2-C is a transparent bottomisometric view with rays. FIG. 2-D is a bottom isometric view withoutrays. FIG. 2 shows in three dimensions the same optical functionality ofthe invention previously shown in two dimensions in FIG. 1-B. FIGS. 1and 2 taken together fully explain the configuration and opticalfunctionality of the preferred embodiment of the invention to one ofordinary skill in the art of traffic stripes.

The thin transparent polymer film with embossed light turning prisms 1on the top surface and with embossed cube corner prisms 2 on the bottomsurface can be mass produced by at least two different methods. In thefirst method, two separate embossed films are made, one including thetop light turning prisms 1 and the other including the bottom cubecorner prisms 2. These two separate embossed films are then laminatedtogether using a transparent bonding agent. This transparent bondingagent may be an adhesive such as pressure sensitive adhesive (PSA) or aliquid adhesive, a solvent which temporarily softens one or both of thesurfaces to be joined, or an alternative agent. Many such bonding agentsare known to those of ordinary skill in the art of laminating polymerlayers together. In the second method, a single embossed film includesthe light turning prisms 1 on the top surface and the cube corner prisms2 on the bottom surface. The first method would allow the use ofcommercially available prismatic sheet for the bottom film, while thesecond method would be more cost effective in the long term.

FIG. 3 presents more details on the lengthwise distribution of therepeating pattern of light turning prisms 1 for optimal opticalperformance. For the preferred embodiment of the invention, six cubecorner prisms 2 are illuminated of the eight total cube corner prisms 2beneath the tilted surface of each light turning prism 1. To enablethese six cube corner prisms 2 to be illuminated, the spacing betweenlight turning prisms 1 and structural bars 3 must be selected tominimize ray blockage for both incident rays 4 and retroreflected rays5. FIG. 3-A shows two adjacent sets of light turning prisms 1 andstructural bars 3 including the wide spacing between these sets tominimize ray blockage. FIG. 3-B shows the optical functionality of oneset of light turning prisms 1, structural bar 3, and cube corner prisms2. Incident rays 4 and retroreflected rays 5 overlap one another as theyproceed in opposite directions. FIG. 3-C shows the next adjacent set oflight turning prisms 2 and structural bar 3. The spacing between setshas been chosen to prevent ray blockage by the adjacent set, as shown bythe incident rays 4 and retroreflected rays 5 as they pass unimpededover structural bar 3 in FIG. 3-C. The proper spacing between adjacentsets of light turning prisms 1 and structural bars 3 to minimize rayblockage for both incident rays 4 and reflected rays 5 whileilluminating the proper number of cube corner prisms 2 is anothercritical feature of the invention.

FIG. 4 shows the same information as FIG. 3 for incident rays 4 arrivingfrom the left instead of the right. FIGS. 3 and 4 taken together showthe bi-directional functionality of the preferred embodiment of theinvention.

FIGS. 5 through 8 present results of a parametric optical analysis bythe inventor to determine the critical design rules for the invention tomaximize performance. This parametric analysis led to the preferredembodiment of the invention and its unprecedented performance, whichcorresponds to 1,000 times the retroreflective brightness of the currentstate of the art. FIGS. 5 through 8 correspond to an increasing numberof cube corner prisms 2 under the tilted surface of each light turningprism, from one to two to four to eight, respectively, of such cubecorner prisms 2 under the tilted surface of each light turning prism 1.

Each of FIGS. 5 through 8 shows four different ray trace cases. TheA-view shows the case where the cube corner prism(s) 2 are perfectlyaligned with the light turning prism 1, which is fully illuminated fromtop to bottom of its vertical face. The B-view shows the case where thecube corner prism(s) 2 are perfectly misaligned with the light turningprism 1, which is fully illuminated from top to bottom of its verticalface. The C-view shows the case where the cube corner prism(s) 2 areperfectly aligned with the light turning prism 1, which is illuminatedonly over the top half of its vertical face. The D-view shows the casewhere the cube corner prism(s) 2 are perfectly misaligned with the lightturning prism 1, which is illuminated only over the top half of itsvertical face.

Bundles of incident rays 4 and reflected rays 5 are shown for all fourcases (A, B, C, D) on all four figures (FIGS. 5, 6, 7, 8). Differenthatch patterns are used for incident ray bundles and reflected raybundles in each figure. The incident rays arrive at a grazing incidenceangle of 1.24 degrees off horizontal, which corresponds to illuminationby headlights 0.65 meters above the road 30 meters away. In other words,arctangent (0.65/30)=1.24 degrees. This angle corresponds to thestandard test angle used in measuring the coefficient ofretroreflectivity of road stripes in the U.S. and much of the world, aswill be discussed in more detail below.

As discussed in the previous section, fractional losses of reflectedrays compared to incident rays are due to one unusual aspect ofreflection from cube corner prisms 2. Rays that enter the right side ofthe prism depart from the left side of the prism, and vice versa. Thiscauses a substantial offset in position for the reflected rays 5compared to the incident rays 4. This offset can lead to major losses ofreflected rays 5 compared to incident rays 4. One type of loss isvertical escape of the reflected rays 5 from the prismatic structure.Another type of loss is blockage of reflected rays 5 by the nextadjacent set of light turning prisms 1 and structural bar 3. The impactof these losses can be devastating to the optical performance. Forexample, FIG. 1-A shows no lost rays for one perfectly aligned cubecorner prism 2 under the tilted surface of one light turning prism 1with full illumination of the vertical face of the light turning prism1. But FIG. 1-B shows that a misaligned cube corner prism 2 causes 100%loss of reflected rays for this same fully illuminated light turningprism 1. Furthermore, FIG. 1-C shows 100% loss of reflected rays 5 for ahalf-illuminated light turning prism with a perfectly aligned cubecorner prism 2 beneath the tilted surface of the light turning prism 1.Still furthermore, FIG. 1-D shows 100% loss of reflected rays 5 for ahalf-illuminated light turning prism with one misaligned cube cornerprism under the tilted surface of the light turning prism.

As the number of cube corner prisms 2 beneath each light turning prism 1increases, the lost ray fraction for the four cases in each of FIGS. 6,7, and 8 improves. The table below summarizes the key results of theparametric study presented in FIG. 5-8. The average lost ray fraction isestimated by averaging the best case (A and C views of FIGS. 5-8) andworst-case lost ray fractions (B and D views of FIGS. 5-8).

Number Worst Case Worst Case of Cube Lost Ray Lost Ray Average LostAverage Lost Corners Fraction Fraction Ray Fraction Ray Fraction Beneathfor All Cube for Half of for All Cube for Half of Sloped Corners CubeCorners Corners Cube Corners Surface Illuminated Illuminated IlluminatedIlluminated 1 100% 100%   50%  100% 2  50% 100%   25%   50% 4  25%  50%12.5%   25% 8  13%  25% 6.25% 12.5%

The average lost ray fraction values are the most important values inthe table above because precise alignment between the tilted surface ofeach light turning prism 1 and the cube corner prisms 2 below thistilted surface is not practical for low-cost, roll-to-roll embossing ofthe polymer film or films used to produce the invention. The combinederrors of diamond-turned master tools, electroformed replica productiontools, assembled drums of replica tools used in the polymer embossingprocess, and the thermal polymer embossing process itself will not allowprecise alignment between the tiny microstructured prisms over the greatlengths of embossed traffic stripe film. For example, in the preferredembodiment, each cube corner prism is about 90 micrometers long. Arepeating error of 1 micrometer in position for each cube corner prismrelative to the tilted surface above it would correspond to a cumulativepositional error of over 1 cm in just a meter of length of trafficstripe, and over 1 meter for a typical 100 meter roll of traffic stripematerial. Similarly, a repeating error of 0.1 micrometer in relativeposition for each cube corner prism would correspond to a cumulativepositional error of over 0.1 cm in just a meter of length of trafficstripe, and over 10 cm for a typical 100 meter roll of traffic stripematerial. Since the relative difference in alignment between perfectalignment and perfect misalignment is only one half of the length of onecube corner prism or 45 micrometers, it is not practical to ensureperfect alignment between the cube corner prisms and the tilted surfaceof the light turning prism above. Therefore, the average lost rayfraction values in the table above will best characterize actualperformance of the invention.

FIG. 9 shows the data in the table above in plotted graphical form. FIG.9-A shows the worst-case loss for perfectly misaligned cube cornerprisms 2 under the tilted face of the light turning prism 1 for bothfully illuminated and half illuminated cases. FIG. 9-B shows the averageloss between perfectly aligned and perfectly misaligned cube cornerprisms 2 under the tilted surface of light turning prism 1 for bothfully illuminated and half illuminated cases. The ordinate in the graphsof FIG. 9 is lost ray fraction for reflected rays 5 compared to incidentrays 4. The abscissa is total number of cube corner prisms 2 under thetilted surface of light turning prism 1. The results in FIG. 9-B bestrepresent the actual performance of the invention as discussed abovesince alignment of cube corner prisms 2 and the tilted surface of thelight turning prisms 1 will vary from worst case to best case over shortlengths of traffic stripe.

Note in FIG. 9-B that large numbers of cube corner prisms 2 under thetilted surface of light turning prisms 1 are essential to minimizelosses. To achieve the desired 1,000X brightness advantage of theinvention, such losses should be kept below 10%. This implies that sixor more cube corner prisms 2 should be employed below the tilted surfaceof each light turning prism 1 even for full illumination, and even morecube corner prisms 2 are needed for less than full illumination.

Note that the lost ray fraction is smaller for full illumination thanfor half illumination. However, as will be discussed below, fullillumination is not desirable from practical considerations of dirtbuildup in the valleys of the exposed prismatic structure on the uppersurface of the invention.

After consideration of the performance losses in FIG. 9-B and practicalfactors such as dirt buildup on the invention, the preferred embodimentuses eight cube corner prisms under the tilted surface of each lightturning prism 1, and three-quarter illumination corresponding to sixilluminated cube corner prism 2 under each light turning prism 1. Thedata point in FIG. 9-B using the symbol X shows the performance of thepreferred embodiment, corresponding to about 8.3% lost ray fraction. Thepreferred embodiment will meet the 1,000X performance advantage goal ofthe invention.

FIG. 10 shows some practical considerations which lead to the preferredembodiment of the invention described above and shown in FIG. 10. FIG.10-A shows the preferred embodiment in pristine new form. FIG. 10-Bshows the preferred embodiment after traffic damage and dirt buildup inthe field. Incident rays 4 and reflected rays 5 are shown in both views.

FIG. 10-B shows traffic damage as the rounding away of the sharp cornerson the top of the prismatic structure. The rounded and partially missingcorners 8 cause a loss in performance which is shown schematically byfewer successfully reflected rays 5. The incident rays 4 which are notsuccessfully reflected by the rounded prismatic structure have beeneliminated from FIG. 10-B for clarity. If 75% of the height of the lightturning prism 1 remains functional after traffic abrasion, the inventionwill still provide 67% of its pristine new performance.

FIG. 10-B shows dirt buildup 10 in all the valleys between exposedprismatic structures. If such dirt buildup 10 only causes opticalblockage and loss for 25% of the height of the light turning prism 1, nofurther loss will occur for the preferred embodiment which only uses thetop 75% of the light turning prism. This is the justification for theselected design illumination value of 75% for the preferred embodiment.Note that if the pristine preferred embodiment in FIG. 10-A achieves1,000X advantage in retroreflected brightness over the present state ofthe art in traffic stripes, then the damaged and dirty preferredembodiment in FIG. 10-B will still retain a 667X advantage over thepresent state of the art.

FIG. 11 summarizes the critical design features of the invention asdiscovered by the inventor. FIG. 11-A shows the side view of theinvention including the critical spacing S between repeating patterns oflight turning prisms 1 and structural bars 3. FIG. 11-B shows the sameview including incident rays 4 and reflected rays 5. Note that theproper selection of the spacing parameter S will minimize blockage ofincident rays 4 and reflected rays 5 while providing the illumination ofthe desired number of cube corner prisms 2.

FIG. 11-C shows a blowup of the optically functional prismaticstructure, while FIG. 11-D shows the definitions of the critical generaldimensions of this prismatic structure. The design rules for excellentoptical performance are summarized in the rectangular box in FIG. 11.While the preferred embodiment comprises eight cube corner prisms 2below the tilted face of each light turning prism 1 and six illuminatedcube corner prisms 2 provided by the appropriate spacing S betweenrepeating patterns of light turning prisms 1 and raised structural bars3, the design rules cover slightly lower performing embodiments of theinvention which will still provide much higher performance than thecurrent state of the art traffic stripes. These design rules will becited in the claims stated below.

FIG. 12 provides the specific dimensions of the preferred embodiment ofthe invention in sufficient detail to enable one of ordinary skill inthe art of microstructured polymer embossing to practice the invention.FIG. 12-A shows a side view of the invention including incident rays 4and reflected rays 5. FIG. 12-B shows the same side view with therequired spacing S of 3.55 cm to minimize blockage of incident rays 4and reflected rays 5 by the adjacent set of light turning prisms 1 andstructural bars 3 for the preferred embodiment. FIG. 12-B also includesblow up views of the dimensions of the critical elements of thepreferred embodiment. Selection of these dimensions by the inventor wasmade to enable extrusion embossing of a polymer sheet with a startingthickness of about 0.075 cm before the prismatic structures are embossedon the top and bottom surfaces. Such polymer sheets are available inroll form from a variety of vendors.

The spirit and scope of the invention is in no way limited to thespecific dimensions shown in FIG. 12. The same performance can beachieved by scaling these specific dimensions either upward or downwardin scale. The key elements of the invention are the relative dimensionsof the repeating prismatic structures on the top and bottom surfaces,and the wide spacing between these repeating patterns. These keyelements of the invention include: (1) a significant number of cubecorner prisms 2 are present below the tilted surface of each lightturning prism 1, (2) a substantial portion of these cube corner prisms 2are illuminated by approaching headlights, and (3) the spacing S betweenrepeating structures is large enough to enable the illumination of asubstantial portion of the cube corner prisms 2 while minimizingblockage of incident rays 4 and reflected rays 5.

FIG. 13 shows why the invention should achieve 1,000X higherretroreflective brightness than the current state of the art for trafficstripes. The top half of FIG. 13 describes the current target value of100 mcd/m²-lux for road marking stripes proposed by the Federal HighwayAdministration (FHWA) which has been adopted by various states, andshows photos of road marking stripes of values close to this target. Asseen in the photos, this target is not extremely bright, but it is stilldifficult to achieve and maintain with the present state of the art intraffic stripes. In contrast to road marking stripes, road signstypically offer more than 1,000X brighter retroreflective brightness assummarized in the table at the lower right of FIG. 13. The reason roadsigns can be made so much brighter is due to the much more normalincidence angle of the light from headlights onto signs compared to roadmarkings. Cube corner retroreflective prismatic sheet works extremelywell for near normal incidence light, but does not work for grazingincidence angle light. This explains why this prismatic retroreflectivesheet technology used in road signs is not employed for road markings.The present invention overcomes this problem by changing the grazingincidence angle light from headlights into near normal incidence anglelight onto the cube corner retroreflective prisms. The invention is thusa game changer in terms of enabling the advanced technology which isused all over the world in road signs to be used in traffic stripes. Thewidely space light turning prisms 1 essentially transform the grazingincidence angle rays 4 into near normal rays from the perspective of thecube corner prisms 2 below.

The table in the lower right of FIG. 13 summarizes the retroreflectivebrightness of Type XI retroreflective sign sheeting for a variety ofobservation angles up to 1 degree. All of these values exceed 120,000mcd/m²-lux. As discussed above, the preferred embodiment of theinvention limits blockage losses to less than 10%. Therefore, theretroreflective brightness of the new traffic stripes should be morethan 90% times 120,000 mcd/m²-lux which is well over 100,000 mcd/m²-lux,more than 1,000X higher than the targeted minimum value at the top ofFIG. 13.

While the greatest market for the invention is expected to be roadmarking stripes, including lane delineation stripes, road edge stripes,crosswalk stripes, and intersection stripes, many other applicationswill also be identified by those of ordinary skill in the art. Theseother applications fall within the spirit and scope of the invention.For example, the new traffic stripe can also be used on verticalsurfaces such as concrete barriers and guardrails, as shown in FIG. 14.For these applications, the invention can be used in a tape form tominimize installation time and cost. The embossed polymer film withlight turning prisms on one side and cube corner prisms on the oppositeside can be unrolled and pressed against the concrete barrier orguardrail using a pressure sensitive adhesive (PSA) to provide theattachment. In essence, the installer will unroll a strip of trafficstripe tape, stick it onto the vertical structure, and walk away withthe job done. These vertical applications are simpler and less demandingthan road stripe applications, since no traffic damage will occur, andno rainwater runoff problems will be encountered for the former.

FIG. 15 shows the geometry of the standard test used in most countriesto measure the retroreflective brightness of traffic stripes. In theU.S., this test is specified in ASTM 1710. The light source simulatesheadlights 0.65 meters above the road surface and 30 meters distant,together defining the 1.24-degree grazing ray angle relative tohorizontal. In terms of the incidence angle of the light relative to asurface normal to the traffic stripe, the complementary angle of 88.76degrees is the proper value. At such a high incidence angle, theretroreflective sheeting used in road signs will not work by itself fortwo reasons: (1) almost all the incident light will be reflected away atthe top surface of the sheeting, and (2) any remaining light transmittedinto the sheeting will not be properly retroreflected by the cube cornerprisms due to the extremely high incidence angle.

Note also in FIG. 15 that the measurement point for retroreflected lightis 1.2 meters above the road, where the driver of the vehicle is assumedto be located. Since the arctangent of 1.2/30=2.29 degrees and thearctangent of 0.65/30=1.24 degrees, the angle between the incident raysand the reflected rays to be measured is about 1.05 degrees. Thisobservation angle is very close to the value in the fourth row of thetable in FIG. 13 for road sign sheeting. Since the invention essentiallychanges the illumination angle from grazing to normal as far as the cubecorner prisms are concerned, the expected retroreflective brightness ofthe invention will be within 10% of the 120,000 mcd/m²-lux value in thatsame fourth row of the table in FIG. 13.

The lower portion of FIG. 15 summarizes the target value ofretroreflective brightness by the FHWA for highways with speeds above 70miles per hour: 100 mcd/m²-lux. As discussed above, the invention willprovide 1,000X this target value.

The target retroreflective brightness value for the invention of 1,000Xthe current targeted value of 100 mcd/m²-lux may seem excessive withoutfurther explanation. The inventor's rationale is two-fold. The muchbrighter traffic stripes will be much more visible to drivers ofdifferent types of vehicles from greater distances. This will reducelane departure accidents and the serious injuries and deaths that oftenaccompany these types of accidents. This first rationale is of greatestimportance. A second rationale is that a 1,000X brighter traffic stripewill retain its life-saving brightness longer than conventional trafficstripes. Consider conventional road stripes which often degrade belowthe targeted value in months or very few years due to traffic damage andnormal environmental degradation. For example, if a conventional roadstripe has an exceptional brightness of 250 mcd/m²-lux when firstinstalled, but degrades in brightness by 10% per month in the field, itsremaining brightness will be months reduced by a factor of about0.9^(months). After 12 months, the reduction will be to about 28% of itsinitial brightness or 71 mcd/m²-lux. This value is below the targetedvalue. In contrast, if the same degradation rate applied to the presentinvention, which has an assumed initial brightness of 100,000mcd/m²-lux, the reduction after 12 months will be to about 28,000mcd/m²-lux, still 280 times the targeted value. Even after 48 months,the brightness of the invention will only fall to about 600 mcd/m²-lux,still six times higher than the target value. While we do not know theactual rate of degradation for the invention, which will no doubt varywidely with location, the higher initial brightness will allow muchgreater degradation over longer periods of time while still maintaininga brightness higher than the targeted values. Extending the lifetime ofthe traffic stripe will reduce life cycle costs, which include periodicreplacement costs, while also saving lives over the full extendedlifetime.

The invention employs a repeating pattern of widely spaced light turningprisms 1 on the top surface and an array of cube corner prisms 2 on thebottom surface of a transparent polymer sheet. Cube corner prisms havebeen used as traffic safety retroreflectors for about a century as shownfor example by U.S. Pat. No. 1,671,086 in 1928. In recent decades, suchcube corner prisms have been used in mass produced road sign sheetingwhich has evolved through various generations into a veryhigh-technology family of products by major corporations including 3Mand Avery Dennison. The brightest of such cube corner sheeting for roadsigns is classified as Type XI under ASTM D49562, as shown in FIG. 13.The brightest sheeting uses “full cube” cube corner reflective prisms.3M calls their family of “full cube” retroreflective sheeting productsDiamond Grade 3 ®. Avery Dennison calls their family of “full cube”retroreflective sheeting products Omnicube®. The preferred embodiment ofthe present invention will use “full cube” prisms on the bottom surfaceto maximize optical performance.

FIGS. 16 and 17 describe “full cube” prisms in more detail, since theyare preferred for the invention. The invention will work with earlierforms of cube corner prisms, which clearly fall within the scope andspirit of the invention, but “full cube” technology is preferred formaximum retroreflective brightness.

FIG. 16-A shows a cube corner from the perspective of looking directlyinto the cube corner. The light gathering aperture is triangular inshape, and the three back faces are mutually orthogonal since they forma cube corner. When light enters the triangular aperture, it will beretroreflected only if it makes three successive reflections off thethree back faces. Not all the light entering the triangular aperturemakes three reflections. The corners of the triangular aperture are deadareas which do not retroreflect because light entering those areas doesnot make all three reflections. FIG. 16-B shows the dead areas in black.Full cube technology uses only a rectangular portion of the cube corneras shown in FIG. 16-C. This rectangular portion is called a full cube inthe literature, since it fully retroreflects light entering itsrectangular aperture. For ease in toolmaking, two full cubes are oftenused as a pair as shown in the lower right drawing of FIG. 16. Many suchpairs of full cubes are then arrayed into sheet form as shown in thelower left drawing of FIG. 16. Such full cube sheets comprise thebrightest available Type XI road sign sheeting.

FIG. 17 shows the full cubes in three-dimensional views. FIG. 17-A showsa pair of full cubes. FIG. 17-B shows the same pair of full cubes intransparent form with a few of the incident rays 4 and retroreflectedrays 5. FIG. 17-C shows an array of pairs of full cubes from one view,while FIG. 17-D shows an array of pairs of full cubes from another view.

To perform their total internal reflection (TIR) function, the fullcubes must be surrounded by air or another gas or vacuum with arefractive index of about 1.0. Manufacturers of full cube sheeting forroad signs provide the needed air pockets in clever ways, generallyusing a printed honeycomb pattern of sealing bonds between the cubecorner prismatic sheet and a separate film thereby creating air pocketsin the individual honeycomb regions. FIG. 18 shows two of the leadingproduct families of Type XI retroreflective sheeting for road signs.

FIG. 18-A is excerpted from the 2014 FHWA guide retroreflective sheetingidentification guide. The 3M product family is known as Diamond Grade 3® and the Avery Dennison product family is known as Omnicube®. These twoproduct lines have slightly different honeycomb patterns formed by theseals around air pockets. FIG. 18-B shows an enlarged photo of the 3Mproduct highlighting the seals and the full cube prisms. The seals blocka large fraction of the area of the cube corners, causing proportionallosses in retroreflectivity. The large loss is due to the small size ofeach air pocket, typically a few millimeters in size.

This small size is required in road sign sheeting because road signscome in so many different sizes and shapes and the cube corner sheetingmust be able to be trimmed to this wide variety of size and shapes.Larger air pockets would not be practical, since trimming destroys theair pockets along the trimmed edges, and the lost edge areas must besmall relative to the sign area to maintain acceptable performance andappearance of the sign. For the present invention, the traffic stripeswill be fixed in width with no trimming needed in this dimension.Furthermore, the traffic stripes will typically be long in length.Therefore, the air pockets can be much larger for the present inventionthan for road sign sheeting. This is important since the sealing bondsaround the small air pockets in road sign sheeting typically obscuresubstantial portions of the cube corners, resulting in an optical lossof 20-30%. For the present invention, larger air pockets can reduce thisloss substantially in a fully optimized embodiment of the invention.

The preferred embodiment of the invention will utilize air pocketsbetween the cube corner prisms 2 and an underlying film which may bewhite to provide daytime visibility for the traffic stripe. Thepreferred embodiment may also include a colored pigment in the prismaticpolymer sheet. For example, a yellow pigment can be used for yellow lanestripe applications and a red pigment may be used on certain guard railsstripes, if such colors are desired. Such colored pigments are alreadyused in Type XI road sign retroreflective sheeting.

The invention will also typically employ additional layers beneath thefilm which provides the air pockets facing the cube corner prisms 2.Such additional layers may include pressure sensitive adhesive (PSA) tofacilitate attachment to a highway or a guardrail. Such additionallayers may include compliant layers to mitigate traffic damage to theprismatic structures used in the invention.

There are at least two acceptable methods of mass producing theprismatic polymer film employed in the invention. One method is to firstproduce a separate prismatic film containing the repeating pattern oflight turning prisms 1 on one side of this first film, with the oppositeside smooth and planar. The smooth and planar surface of the first filmis then bonded to the smooth and planar upper surface of existing TypeXI reflective sheeting described above using a transparent bonding agentsuch as solvent or liquid adhesive or pressure sensitive adhesive. Thefirst film with light turning prisms 1 is produced by embossing atransparent polymer film, the same embossing method commonly used toproduce the Type XI reflective sheeting. An alternate method of massproducing the prismatic polymer film is to simultaneously emboss bothsets of prisms, namely the light turning prisms 1 on one side of thefilm and the cube corner prisms 2 on the opposite side of the film. Thisalternate method will require two embossing rolls corresponding to thetwo prismatic patterns rather than the normal one embossing with aprismatic pattern and another roll will a polished surface. The inventorhas had discussions with long-time suppliers of prismatic sheeting andfound that the simultaneous embossing is practical and cost-effective.The first method of producing the prismatic polymer film offers thebenefit of using a proven product for the cube corner retroreflectors inthe short term, but the second method will be more cost-effective in thelong term.

While the above paragraphs have fully described the invention and itsbest mode of implementation so that one of ordinary skill in the art canpractice the invention, many other variations and embodiments of theinvention will become apparent to others of ordinary skill in the artbased upon the disclosure of this invention. Such variations andembodiments fall within the scope and spirit of the invention.

I claim:
 1. A retroreflective road stripe configured to be horizontallyattached to a highway, said road stripe having a length parallel to thedirection of traffic and a width perpendicular to the direction oftraffic, said road stripe comprising the following: a. A substantiallytransparent polymeric material having an upper surface exposed to theambient environment and a lower surface facing said highway below, b.Said upper surface including a plurality of linear light-turning prismsextending substantially across the width of said road stripe in arepeating pattern along the length of said road stripe, each of saidlight-turning prisms having three exposed faces, i. The first of saidexposed faces is substantially vertical, facing oncoming traffic ii. Thesecond of said exposed faces is opposite said first face and inclined atsubstantially 45 degrees relative to horizontal, and having a horizontallength, L1 iii. The third of said exposed faces is substantiallyhorizontal connecting the tops of said first and second faces and havinga horizontal length, L3 c. Said lower surface including a plurality ofcube-corner retroreflective prisms, i. Said cube-corner prisms eachhaving a horizontal length, L2 ii. Said cube-corner prism horizontallength, L2, being at least 83% smaller than the said horizontal length,L1, of said second exposed face of said light-turning prisms iii. Saidcube-corner retroreflective prisms being surrounded by air spaces belowsaid prisms to enable total internal reflection by said prisms d. Saidrepeating pattern of said light-turning prisms spaced apart along thelength of said road stripe by a spacing, S, which obeys the inequalitybelow:${S \geq {\frac{3\mspace{11mu} L\; 2}{\tan\left( {1.24{^\circ}} \right)} + {L1} + {L3}}}.$2. The retroreflective road stripe of claim 1 comprising repeating pairsof said light-turning prisms facing in both directions of traffic. 3.The retroreflective road stripe of claim 1 wherein said polymericmaterial is selected from acrylic, polycarbonate, urethane, silicone,fluoropolymer, polyester, and combinations thereof.
 4. Theretroreflective road stripe of claim 1 wherein said light-turning prismsand said cube-corner prisms are formed into said polymeric material by aroll-to-roll embossing process of moldable polymer material proceedingbetween embossing rolls containing the reverse prismatic patterns to beembossed into said polymer film.
 5. The retroreflective road stripe ofclaim 1 wherein said road stripe further comprises taller shoulderstructures proximate to the two long edges of said road stripe tomitigate traffic damage to said road stripe.
 6. The retroreflective roadstripe of claim 1 wherein said road stripe further comprises compliantmaterials beneath said road stripe to mitigate traffic damage to saidroad stripe.
 7. The retroreflective road stripe of claim 1 wherein saidroad stripe further comprises an enclosed air gap beneath saidretroreflective cube corner prisms to promote total internal reflectiontherefrom.
 8. A retroreflective road stripe configured to behorizontally attached to a highway, said road stripe having a lengthparallel to the direction of traffic and a width perpendicular to thedirection of traffic, said road stripe comprising the following: a. Asubstantially transparent polymeric material having an upper surfaceexposed to the ambient environment and a lower surface facing saidhighway below, b. Said upper surface including a plurality of linearlight-turning prisms extending substantially across the width of saidroad stripe in a repeating pattern along the length of said road stripe,each of said light-turning prisms having three exposed faces, i. Thefirst of said exposed faces is substantially vertical, facing oncomingtraffic ii. The second of said exposed faces is opposite said first faceand inclined at substantially 45 degrees relative to horizontal, andhaving a horizontal length, L1 iii. The third of said exposed faces issubstantially horizontal connecting the tops of said first and secondfaces and having a horizontal length, L3 c. Said lower surface includinga plurality of cube-corner retroreflective prisms, i. Said cube-cornerprisms each having a horizontal length, L2 ii. Said cube-corner prismhorizontal length, L2, being at least 83% smaller than the saidhorizontal length, L1, of said second exposed face of said light-turningprisms iii. Said cube-corner retroreflective prisms being surrounded byair spaces below said prisms to enable total internal reflection by saidprisms, d. Said upper surface of said light-turning prisms accompaniedwith additional taller linear structural elements of height, Hmax, andhorizontal length, L4, located nearby and parallel to said light-turningprisms on the opposite side of said light-turning prisms from said firstvertical face which faces oncoming traffic, e. Said repeating pattern ofsaid light-turning prisms and said taller structural elements spacedapart along the length of said road stripe by a spacing, S, which obeysthe inequality below:${S \geq {\frac{{H\max} - {L\; 1} + {3\mspace{11mu} L\; 2}}{\tan\left( {1.24{^\circ}} \right)} + {L1} + {L3} + {L4}}}.$9. The retroreflective road stripe of claim 8 comprising repeating pairsof said light-turning prisms facing in both directions of traffic. 10.The retroreflective road stripe of claim 8 wherein said polymericmaterial is selected from acrylic, polycarbonate, urethane, silicone,fluoropolymer, polyester, and combinations thereof.
 11. Theretroreflective road stripe of claim 8 wherein said light-turning prismsand said cube-corner prisms are formed into said polymeric material by aroll-to-roll embossing process of moldable polymer material proceedingbetween embossing rolls containing the reverse prismatic patterns to beembossed into said polymer film.
 12. The retroreflective road stripe ofclaim 8 wherein said road stripe further comprises taller shoulderstructures proximate to the two long edges of said road stripe tomitigate traffic damage to said road stripe.
 13. The retroreflectiveroad stripe of claim 8 wherein said road stripe further comprisescompliant materials beneath said road stripe to mitigate traffic damageto said road stripe.
 14. The retroreflective road stripe of claim 8wherein said road stripe further comprises an enclosed air gap beneathsaid retroreflective cube corner prisms to promote total internalreflection therefrom.
 15. A retroreflective traffic safety stripeconfigured to be vertically attached to a highway barrier or guardrailproximate to and parallel to the edge of said highway, said trafficsafety stripe having a length parallel to the direction of traffic and awidth perpendicular to the said length, said traffic safety stripecomprising the following: a. A substantially transparent polymericmaterial having an upper surface exposed to the ambient environment anda lower surface facing said highway barrier beneath, b. Said uppersurface including a plurality of linear light-turning prisms extendingsubstantially across the width of said road stripe in a repeatingpattern along the length of said road stripe, each of said light-turningprisms having three exposed faces, i. The first of said exposed faces issubstantially vertical, facing oncoming traffic ii. The second of saidexposed faces is opposite said first face and inclined at substantially45 degrees relative to said first face, and having a horizontal length,L1 iii. The third of said exposed faces is substantially horizontalconnecting the tops of said first and second faces and having ahorizontal length, L3 c. Said lower surface including a plurality ofcube-corner retroreflective prisms, i. Said cube-corner prisms eachhaving a horizontal length, L2 ii. Said cube-corner prism horizontallength, L2, being at least 83% smaller than the said horizontal length,L1, of said second exposed face of said light-turning prisms iii. Saidcube-corner retroreflective prisms being surrounded by air spaces belowsaid prisms to enable total internal reflection by said prisms d. Saidrepeating pattern of said light-turning prisms spaced apart along thelength of said traffic safety stripe by a spacing, S, which obeys theinequality below:${S \geq {\frac{3\mspace{11mu} L\; 2}{\tan\left( {1.24{^\circ}} \right)} + {L1} + {L3}}}.$16. The retroreflective traffic safety stripe of claim 15 comprisingrepeating pairs of said light-turning prisms facing in both directionsof traffic.
 17. The retroreflective traffic safety stripe of claim 15wherein said polymeric material is selected from acrylic, polycarbonate,urethane, silicone, fluoropolymer, polyester, and combinations thereof.18. The retroreflective traffic safety stripe of claim 15 wherein saidlight-turning prisms and said cube-corner prisms are formed into saidpolymeric material by a roll-to-roll embossing process of moldablepolymer material proceeding between embossing rolls containing thereverse prismatic patterns to be embossed into said polymer film. 19.The retroreflective road stripe of claim 15 wherein said traffic safetystripe further comprises an enclosed air gap beneath saidretroreflective cube corner prisms to promote total internal reflectiontherefrom.